A large chunk of unlicensed spectrum exists in the 2 and 5 GHz spectrum, known to be used by WiFi. However, the spectrum is unlicensed and open for other technologies. In some markets, no requirements exist for Listen-Before-Talk, LBT, to be used. In those markets a duty cycle based transmission may be used, comprising a transmission phase and a silent phase which allow technologies not relying on LBT to transmit. The duty cycled based transmission can start to transmit than someone else is transmitting. To balance the load between different system and technologies a duty cycle based system can use a Carrier-Sensing Adaptive Transmission, CSAT, to adapt the channel usage. To adapt the channel usage, other detected transmissions over a given energy level can be used; which is the Energy Detection, ED. The duty cycle based approach is used by the first LTE versions, LTE-U, operating in the unlicensed spectrum. In order to be able to operate in all markets the function called LBT is required. The usage of LBT means that the channel usage do not have to be adapted by a CSAT approach any longer, instead the system will sense the carrier before it starts to transmit; the transmission starts when it see the channel is free. In a system with mixed technologies an ED thread hold value can be used to determine when the channel is free. When this is used in LTE, it is called License Assisted Access (LAA).
Generally, network nodes try to determine which data rate to send with by some kind of feedback from the receiver. Such methods may be quality feedback, e.g. CQI, or that the network nodes probe which rate to be used. An example is Long Term Evolution, LTE, which uses CQI together with an outer loop. Another example is WiFi which generally uses probing with the only feedback of ACK if the transmission was successful.
Link adaptations, LA, may be used to predict what deviations exist in the CQI compared to the real channel performance of a wireless device, i.e. bases on errors in the transmitted subframes the used MCS can be changed a little based on what was predicted by the CQI.
If LBT is used, other more advanced methods may be used by users to reserve the channel by sending a reservation. WiFi does this with Clear-To-Send, CTS, but this method doesn't work well between different technologies, since the use of unlicensed spectrum can't force a technology to decode what other technologies send.
When only one technology exists in a spectrum it can be designed to time-share the spectrum between users such as WiFi is doing or it can be designed to operate well in an environment with high interference such as WDCMA or LTE. Mixing the two may be somewhat problematic.
In LTE-U (Long Term Evolution Unlicensed) only the downlink is transmitted on the carrier(s) in the unlicensed spectrum while both uplink and downlink are transmitted on the carrier(s) in the licensed spectrum. In LAA either only downlink is or a combination of downlink and uplink are transmitted on the unlicensed spectrum. The carrier(s) in the unlicensed spectrum is called Secondary Cell, SCell, and to a large extent works as it is carrier aggregated with the carrier in the licensed spectrum, Primary Cell, PCell.
LTE is designed to operate with the same frequency in all cells. A cell is the coverage area of a network node. A network node may be associated with more than one cell. However, WiFi prefer to use a sparser frequency reuse since it wants as few strong interferes as possible to get less users to time share the channel with.
When both WiFi and unlicensed LTE are deployed, co-sited or not, WiFi can cause strong interference at the wireless device in bursts which cause problems when LTE-U initiate transmissions without using LBT.
LTE-U is using the channel in a CSAT manner. Meaning that a network node transmits in duty cycles where it transmits for X out of Y subframes. In LTE-U the duty cycle may be 20 subframes (1 subframe is 1 ms) and the network node typically may transmit in between 2-18 of those subframes. In the period the network node is not transmitting it evaluate the carrier and determines how much load (over a given energy level) it is on the carrier from other users. If the load is over a given level the network node may decrease the channel usage and if it is under a given level it may increase the channel usage.
When LBT not is used it exist a risk that a new transmission from a device or node of the LTE-U communication network starts when another node already is transmitting. This may result in the first subframe(s) to be sent meanwhile another transmission already is on-going from another node with strong power and it causes interference. It is first when the already ongoing WiFi interference stops that WiFi does a new clear channel assessment and detects it should not start a new transmission.